It is often desirable to measure stress during the manufacture of pressure vessels, rocket motors, pipes, and other goods which comprise composite materials. One method of making such composite structures is to wind resin-carrying reinforcing fibers in layers around a mandrel. The fibers carry a resin which bonds the fibers together after a curing process. Curing involves polymerization of the resin such that a solid matrix is formed around the fibers.
Typical composite structures are substantially weakened if gaps or other unbonded regions arise between adjacent layers of fiber. In a pressure vessel, such weaknesses can be catastrophic. To assist in obtaining adequate bonding, each new layer of fiber is therefore compressed against the previous layer as it is wound about the mandrel.
Accordingly, it is often desirable to monitor the winding process by measuring the stress between the layers of a composite structure as the fibers are being wound. Additional stress measurements are often taken while the composite structure is being cured. Stress measurements are also used to monitor other processes for manufacturing composite structures, including resin transfer molding, compression molding, pultrusion, and vacuum bagging.
Stress is measured by devices that are configured to measure the force which is acting against an area. Such devices are collectively referred to herein as "stress sensors." Stress sensors may be configured to measure either uniform or nonuniform forces, and may measure compression forces, tension forces, or both. Stress sensors which measure uniform forces per unit area are typically known as pressure sensors or pressure transducers.
A stress sensor suitable for use in monitoring the manufacture of composite structures faces several requirements. In many situations, the desired stress measurements can be obtained only by permanently embedding dozens or even hundreds of stress sensors within each composite structure as the structure is being formed. Thus, stress sensors used in such applications should be relatively inexpensive and easily produced in large numbers.
In addition, stress sensors embedded in composite structures made by fiber winding should be relatively thin. Placing a stress sensor between adjacent layers of fiber in a structure typically causes a "tenting" effect by raising the upper layer above the lower layer. The sensor raises the upper layer not merely where the upper layer rests directly on the sensor, but also in a region around the sensor, thereby creating a gap between the upper and lower layers.
The size of the gap depends largely on the sensor's thickness. Thinner sensors are generally preferable because they reduce the size of the gap. A stress sensor intended for use in monitoring fiber winding should typically be no thicker than about 0.015 inches and preferably no thicker than about 0.010 inches.
Each stress sensor should also be calibrated to provide accurate measurements of stresses in an appropriate range. Stress sensors used to monitor fiber winding and autoclave curing during composite manufacturing should provide measurements in a rated range up to about 400 p.s.i., and preferably in a rated range up to about 500 p.s.i., with an error of no more than about 20 percent of the highest rated output ("full scale" output) without temperature compensation, and preferably with an error of no more than about five percent of full scale output with temperature compensation.
One type of stress sensor known in the art for monitoring fiber winding comprises an arrangement of strain sensitive electrical resistors (strain gauges) secured to a circular diaphragm which is positioned adjacent a cavity. The resistors are configured to receive an electrical energy input and to provide an output voltage, proportional to strain in the diaphragm, which can be read by external equipment. The resistors are arranged on the diaphragm such that movement of the diaphragm into the cavity, in response to a force applied to the sensor, modifies the resistance of the resistors and thereby causes a change in the output voltage. The graph of the output voltage as a function of stress is substantially linear over the rated stress range of the stress sensor. Thus, within the rated range, a change in the output voltage directly corresponds to a change in the stress on the diaphragm.
However, many stress sensors of this general design fail to satisfy the requirements set forth above for stress sensors used to monitor composite manufacturing. Use of such sensors is limited by their thickness, limited accuracy and range, high cost, or some combination of these factors. Some of these drawbacks arise from the methods which are used to make the stress sensors.
For instance, stress sensors that include hand-wired electrical connections are difficult to make in large numbers, and therefore typically cost more per unit than functionally equivalent sensors which were made without hand-wiring. Similarly, manufacturing sensors in groups is preferable to making them one at a time, because more sensors are produced in less time at a lower cost per sensor.
The accuracy of the stress sensor and the range over which the sensor is reliable may also depend on the method used to manufacture the sensor. The behavior curve which tracks the sensor's output voltage as a function of stress is ideally linear, or at least monotonic over some range. To obtain such regular behavior, the diaphragm must move smoothly. Within the specified range, gradual changes in the force against the diaphragm must cause correspondingly gradual changes in the position of the diaphragm. Gradual changes in the diaphragm position produce gradual changes in the resistance of the resistors, resulting in gradual changes in the voltage across the resistors that are secured to the diaphragm.
However, such regular behavior is often difficult to obtain if the electrical resistors are improperly secured or if the diaphragm contains irregularities. Under these circumstances, the diaphragm may lunge or "oil-can." That is, the diaphragm may reach a first position of increased resistance to stress and then lunge suddenly into a distant second position after the stress increases beyond some threshold. A diaphragm that lunges typically fails to produce a regular behavior curve.
Thus, it would be an advancement in the art to provide a method for making stress sensors which are calibrated to provide accurate stress measurements in a range that is appropriate for sensing the stresses present while composite structures are manufactured.
It would be a further advancement to provide such a method for making stress sensors which are thinner than pre-existing stress sensors.
It would also be an advancement in the art to provide such a method for making stress sensors which are less expensive than pre-existing stress sensors.
It would be an additional advancement to provide such a method for easily making a large number of stress sensors.
Such a method is disclosed and claimed herein.